Poly (vinylamine)-based superabsorbent gels and method of manufacturing the same

ABSTRACT

Poly(vinylamine)-based superabsorbent gels are disclosed. The superabsorbent gels either comprise a mixture of a poly(vinylamine) polymer and an acidic water-absorbing polymer, like polyacrylic acid, or comprise a salt of a poly(vinylamine) polymer. An improved method of preparing poly(vinylamine), and improved diaper cores, also are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part application of U.S. application Ser. No.08/974,119, filed Nov. 19, 1997, U.S. Pat. No. 5,981,689.

FIELD OF THE INVENTION

The present invention relates to superabsorbent gels containing apoly(vinylamine), or a salt thereof, and to an improved method ofmanufacturing a poly(vinylamine). The superabsorbent gels comprise apoly(vinylamine), and preferably a poly(vinylamine) admixed with anacidic superabsorbent polymer, like a polyacrylic acid, or comprise asalt of a poly(vinylamine).

BACKGROUND OF THE INVENTION

Water-absorbing resins are widely used in sanitary goods, hygienicgoods, wiping cloths, water-retaining agents, dehydrating agents, sludgecoagulants, disposable towels and bath mats, disposable door mats,thickening agents, disposable litter mats for pets,condensation-preventing agents, and release control agents for variouschemicals. Water-absorbing resins are available in a variety of chemicalforms, including substituted and unsubstituted natural and syntheticpolymers, such as hydrolysis products of starch acrylonitrile graftpolymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonatedpolystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols,polyethylene oxides, polyvinylpyrrolidines, and polyacrylonitriles.

Such water-absorbing resins are termed “superabsorbent polymers,” orSAPs, and typically are lightly crosslinked hydrophilic polymers. SAPsare generally discussed in Goldman et al. U.S. Pat. No. 5,669,894. SAPscan differ in their chemical identity, but all SAPs are capable ofabsorbing and retaining amounts of aqueous fluids equivalent to manytimes their own weight, even under moderate pressure. For example, SAPscan absorb one hundred times their own weight, or more, of distilledwater. The ability to absorb aqueous fluids under a confining pressureis an important requirement for an SAP used in a hygienic article, likea diaper.

The dramatic swelling and absorbent properties of SAPs are attributed to(a) electrostatic repulsion between the charges along the polymerchains, and (b) osmotic pressure of the counter ions. It is known,however, that these absorption properties are drastically reduced insolutions containing electrolytes, such as saline, urine, and blood. Thepolymers do not function as effective SAPs in the presence of suchphysiologic fluids.

The decreased absorbency of electrolyte-containing liquids isillustrated by the absorption properties of a typical, commerciallyavailable SAP, i.e., sodium polyacrylate, in deionized water and in 0.9%by weight sodium chloride (NaCl) solution. The sodium polyacrylate canabsorb 146.2 grams (g) of deionized water per gram of SAP (g/g) at 0psi, 103.8 g of deionized water per gram of polymer at 0.28 psi, and34.3 g of deionized water per gram of polymer of 0.7 psi. In contrast,the same sodium polyacrylate is capable of absorbing only 43.5 g, 29.7g, and 24.8 g of 0.9% aqueous NaCl at 0 psi, 0.28 psi, and 0.7 psi,respectively. The absorption capacity of SAPs for body fluids, likeurine or menses, therefore, is dramatically lower than for deionizedwater because such fluids contain electrolytes. This dramatic decreasein absorption is termed “salt poisoning.”

The salt poisoning effect has been explained as follows.Water-absorption and water-retention characteristics of SAPs areattributed to the presence of ionizable functional groups in the polymerstructure. The ionizable groups typically are carboxyl groups, a highproportion of which are in the salt form when the polymer is dry, andwhich undergo dissociation and salvation upon contact with water. In thedissociated state, the polymer chain contains a plurality of functionalgroups having the same electric charge and, thus, repel one another.This electronic repulsion leads to expansion of the polymer structure,which, in turn, permits further absorption of water molecules. Polymerexpansion, however, is limited by the crosslink in the polymerstructure, which are present in a sufficient number to preventsolubilization of the polymer.

It is theorized that the presence of a significant concentration ofelectrolytes interferes with dissociation of the ionizable functionalgroups, and leads to the “salt poisoning” effect. Dissolved ions, suchas sodium and chloride ions, therefore, have two effects on SAP gels.The ions screen the polymer charges and the ions eliminate the osmoticimbalance due to the presence of counter ions inside and outside of thegel. The dissolved ions, therefore, effectively convert an ionic gelinto a nonionic gel, and swelling properties are lost.

The most commonly used SAP for absorbing electrolyte-containing liquids,like urine, is neutralized polyacrylic acid, i.e., containing at least50%, and up to 100%, neutralized carboxyl groups. Neutralizedpolyacrylic acid, however, is susceptible to salt poisoning. Therefore,to provide an SAP that is less susceptible to salt poisoning, either anSAP different from neutralized polyacrylic acid must be developed, orthe neutralized polyacrylic acid must be modified or treated to at leastpartially overcome the salt poisoning effect.

Prior investigators have attempted to counteract the salt poisoningeffect and thereby improve the performance of SAPs with respect toabsorbing electrolyte-containing liquids, such as menses and urine. Forexample, Tanaka et al. U.S. Pat. No. 5,274,018 discloses an SAPcomposition comprising a swellable hydrophilic polymer, like polyacrylicacid, and an amount of an ionizable surfactant sufficient to form atleast a monolayer of surfactant on the polymer. In another embodiment, acationic gel, like a gel containing quaternized ammonium groups and inthe hydroxide (i.e., OH) form, is used with an anionic gel (i.e., apolyacrylic acid) to remove electrolytes from the solution by ionexchange.

Wong U.S. Pat. No. 4,818,598 discloses admixing a fibrous anion exchangematerial, like DEAE cellulose, and a hydrogel, like a polyacrylate, toimprove absorption properties. WO 96/17681 discloses admixing an anionicSAP, like polyacrylic acid, with a polysaccharide-based cationic SAP toovercome the salt poisoning effect. Similarly, WO 96/15163 disclosesadmixing a cationic SAP having at least 20% of the functional groups ina basic (i.e., OH) form with a cationic exchanges resin, i.e., anonswelling ion exchange resin, having at least 50% of the functionalgroups in the acid form. WO 96/15180 discloses an absorbent materialcomprising an anionic SAP, e.g., a polyacrylic acid and an anionexchange resin, i.e., a nonswelling ion exchange resin.

These references disclose combinations that attempt to overcome the saltpoisoning effect. It would be desirable, however, to provide an SAP thatexhibits exceptional absorbency and retention, like a sodiumpolyacrylate, and, therefore, can be used alone as an SAP. It also wouldbe desirable to admix such an SAP with polyacrylic acid, or anotheracid-containing SAP, to overcome the salt poisoning effect.

SUMMARY OF THE INVENTION

The present invention is directed to poly(vinylamine)-basedsuperabsorbent gels. A poly(vinylamine) polymer can be used inconjunction with an acidic water-absorbing resin, like polyacrylic acid,to help overcome the salt poisoning effect, or a salt of apoly(vinylamine) polymer can be used alone as an SAP. Thepoly(vinylamine) polymer also can be used, alone, as an SAP to absorband retain acidic media. More particularly, the poly(vinylamine) used asan SAP, or as a component of an SAP, is lightly crosslinked and, inpreferred embodiments, is surface treated to improve absorptionproperties.

Accordingly, one aspect of the present invention is to provide animproved method of manufacturing a poly(vinylamine) comprisingvinylamine monomer units, and which can be crosslinked using a suitablepolyfunctional vinyl monomer. The present method substantially reducesthe amount of residual N-vinylamide monomer in the poly(N-vinylamide)precursor of the poly(vinylamine), and, therefore, eliminates thestringent purification procedures, or reduces the long polymerizationreaction times, previously used to overcome the problem of residualmonomer content. Consequently, the present improved process reducesprocess time and production costs.

Another aspect of the present invention is to provide an SAP havingabsorbency and retention properties comparable to a conventional SAP,like sodium polyacrylate. A present SAP is produced by neutralizing apoly(vinylamine) with a sufficient amount of acid, like hydrochloricacid, such that at least about 10%, i.e., about 10% to 100%, of theamine-functional groups are neutralized. The resulting poly(vinylamine)salt is an excellent SAP for absorbing aqueous media.

In accordance with another important aspect of the present invention, alightly crosslinked poly(vinylamine), alone and unneutralized, can beused to absorb and retain acidic aqueous media. The acidic aqueous mediaconverts the low-absorbing poly(vinylamine) to a highly absorbingpoly(vinylamine) salt, i.e., converts the polymer to an SAP, duringabsorption. A poly(vinylamine), therefore, is an excellent resin forcleaning acid spills and the remediation of acidic species.

Yet another aspect of the present invention is to provide an improvedSAP that overcomes the salt poisoning effect of electrolytes. Inparticular, the improved SAP material contains a mixture of an acidicswellable resin, like polyacrylic acid, and a poly(vinylamine).

These and other aspects and advantages of the present invention willbecome apparent from the following detailed description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are plots of acquisition time (seconds) vs. number ofinsults for a series of laboratory prepared diaper cores under a load of0.7 psi; and

FIGS. 3 and 4 are plots of acquisition rate (ml/sec) vs. number ofinsults for a series of laboratory prepared diaper cores under a load of0.7 psi.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to: (a) an improved method ofmanufacturing poly(vinylamine), (b) poly(vinylamine) andpoly(vinylamine) salts and their use as SAPs, and (c) an improved SAPmaterial comprising an admixture of a poly(vinylamine) and an acidicwater-absorbing resin. (a) An Improved Method of ManufacturingPoly(vinylamine)

Poly(vinylamine), and salts derived therefrom, are known polymers. Forexample, the following patents disclose the synthesis or manufacture ofpoly(vinylamine): U.S. Pat. No. 4,798,871; U.S. Pat. No. 4,843,118; andU.S. Pat. No. 4,804,793. In addition, U.S. Pat. No. 4,018,826 disclosesa process for preparing poly(vinylamine) and salts thereof. Ford et al.U.S. Pat. No. 5,491,199 discloses the preparation of formate-freepoly(vinylamine) by heating the polymer in the presence of transitionmetal catalyst.

The above patents generally disclose polymers of N-vinylformamide thatsubsequently are hydrolyzed. Upon hydrolysis, the poly(N-vinylformamide)is converted into a poly(vinylamine). Hydrolysis can be performed underacid or basic conditions. The cationic charge on the resultingvinylamine, i.e., the charge density, is related to the pH of themedium. At a low pH, the poly(vinylamine) is protonated and has a highcationic charge density. Conversely, at a high pH, the poly(vinylamine)is not protonated, and the polymer has a substantially reduced cationiccharge density, if any.

In general, an uncrosslinked poly(vinylamine) is a water-soluble polymerthat has many practical applications, such as in water treatment,personal care products, and ion exchange resins. Poly(vinylamine) isrendered water insoluble by a crosslinking the polymer. Althoughpolyvinylamines, and salts thereof, are well known, it has notheretofore been suggested that such polymers can be used as an SAP.

Typically, a poly(vinylamine) polymer is produced by hydrolysis ofpoly(N-vinylformamide), under either acid or basic conditions.Poly(vinylamine) also can be produced from other poly(N-vinylamides),like poly(N-vinylacetamide), poly(N-vinylpropionamide), andpoly(N-vinylsuccinamide). It is desirable that hydrolysis of thepoly(vinylamide) is substantially to essentially complete, i.e., about10% to 100% complete, and preferably about 30% to 100% complete. Toachieve the full advantage of the present invention, at least about 50%,and more preferably at least about 90%, of the amide groups arehydrolyzed to an amine functionality. The amine-functional polymer cancontain other copolymerizable units, i.e., other monoethylenicallyunsaturated monomers, as long as the polymer is substantially, i.e., atleast 10%, and preferably at least 25%, vinylamine units. To achieve thefull advantage of the present invention, the polymer contains at least50%, and more preferably at least 75%, vinylamine units.

If residual monomer or other impurities are present in thepoly(vinylamide), hydrolysis conditions can lead to a crosslinking,which increases the molecular weight of the poly(vinylamine) and canresult in undesirable and unpredictable gel formation. Therefore,current methods of synthesizing poly(vinylamine) require either arigorous purification of the poly(N-vinylformamide), or an extremelylong reaction time and a relatively high reaction temperature to ensurethat all the residual poly(N-vinylformamide) monomer is consumed duringthe polymerization.

The production of poly(vinylamine) would be facilitated, and productioncosts decreased, by an improved method of removing residual N-vinylamidemonomers from the poly(N-vinylamide). Therefore, in accordance with animportant feature of the present invention, an improved method ofmanufacturing poly(vinylamine) is disclosed.

As set forth above, polymerization of N-vinylformamide, followed byhydrolysis, is the most common method of producing poly(vinylamine). Thepolymerization can be performed in the presence or absence of acrosslinker, i.e., a polyfunctional organic compound. However, residualN-vinylformamide monomer, or other monomer impurities, like aldehydes,can cause crosslinking and undesired gel formation during hydrolysis. Inaccordance with an important feature of the present invention, it hasbeen found that the problem of residual monomer content, and thepresence of other impurities, can be overcome by the use of suitablescavenging agents to remove the residual monomer and other impuritiesfrom the poly(N-vinylamide). The use of scavenging agents has theadvantage of greatly reducing the process time, and costs, currentlyinvested to insure that all the N-vinylamide monomer and otherimpurities are consumed prior to hydrolysis.

In accordance with an important feature of the present invention, ascavenging agent is added to a poly(N-vinylamide), prior to hydrolysis,in an amount of about 0.1% to about 3%, and preferably about 0.1% toabout 2%, by weight, based on the weight of N-vinylamide monomer used inthe polymerization. To achieve the full advantage of the presentinvention, the scavenging agent is added in an amount of about 0.1% toabout 1%, by weight, based on the weight of N-vinylamide monomer.

The scavenging agent can be any compound capable of reacting withN-vinylamides, like N-vinylformamide, and other aldehydic impurities,like formaldehyde or acetaldehyde, under hydrolysis conditions, i.e., atemperature of about 25° C. to about 80° C. for about 4 to about 24hours in the presence of an acid or a base. Typically, a scavengingagent is capable of reacting with an aldehyde in about 1 minute to about10 minutes at a temperature of about 20° C. to about 80° C.

Examples of scavenging agents include, but are not limited to: (a)oxidizing agents, like potassium permanganate, ammonia silver salts(Tollen's Reagent), potassium dichromate, and hydrogen peroxide; (b)reducing agents, like catalytic hydrogenation, lithium aluminum hydride,sodium borohydride, diborane, aluminum hydride, LiAlH(O.t-Bu)₃ (lithiumaluminum tri-t-butoxy hydride), LiAlH-(OCH₃)₃ (lithium aluminumtrimethoxy hydride), zinc (mercury) and concentrated hydrochloric acid,and hydrazine and a base; (c) Grignard reagents, like aryl and alkylmagnesium halides; (d) sodium or potassium cyanide with sodiumbisulfite; (e) sodium bisulfite; and (f) ammonia derivatives, likehydroxylamine, hydrazine, substituted hydrazines, e.g., phenylhydrazine, and semicarbazine. A reducing agent is a preferred scavengingagent, and sodium borohydride is a most preferred scavenging agent. Suchscavenging agents have the advantages of being inexpensive, greatlyreducing the reaction time to form a poly(N-vinylamide), and eliminatingthe need to purify the poly(N-vinylamide).

To achieve the full advantage of the present invention, the scavengingagent is an aqueous solution containing sodium borohydride, e.g., about10% to about 15% by weight, and sodium hydroxide. The sodium borohydrideacts quickly, is highly effective, and is inexpensive. As an addedadvantage, the sodium hydroxide is useful in a subsequent basichydrolysis of the poly(N-vinylamide). Prior to hydrolyzing thepoly(N-vinylamide), the poly(N-vinylamide) and scavenging agent are heldat about 25° C. to about 80° C. for about 1 minute to about 10 minutesto eliminate essentially all, i.e., about 95% to 100%, of the residualmonomers and impurities.

After using a scavenging agent to remove residual monomers and otherimpurities, the poly(N-vinylamide) is hydrolyzed. The amount of acid orbase used to hydrolyze the poly(N-vinylamide) in solution can varywidely, and is generally added in a molar ratio of acid or base toN-vinylamide monomer content of the initially formed polymer of about0.05:1 to about 3:1, preferably of about 0.3:1 to about 1:1. To achievethe full advantage of the present invention, the molar ratio of acid orbase to N-vinylamide monomer is about 0.7:1 to about 1:1.

Generally, hydrolysis is achieved with a suitable acid, such as aninorganic acid, for example, hydrochloric acid, hydrobromic acid,hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, and thelike. In addition, suitable bases, such as an inorganic base, forexample, sodium hydroxide, ammonia, ammonium hydroxide, potassiumhydroxide, and the like, can also be used. Hydrolysis is conducted at atemperature of about 25° C. to about 100° C. for about 4 to about 24hours.

The degree of hydrolysis is controlled by the amount of acid or base,the reaction temperature, and/or the reaction time. In general, greateramounts of acid or base, higher reaction temperatures, and longerreaction times result in higher degrees of hydrolysis.

The present method, therefore, is an improved method of manufacturingeither crosslinked or uncrosslinked poly(vinylamine). The followingexamples illustrate the improved method in the manufacture ofuncrosslinked poly(vinylamine).

EXAMPLE 1

N-vinylformamide (400 g, 5.6 mole) was dissolved in 3,000 g of deionizedwater, then the resulting monomer solution was sparged with argon forone hour. In a separate vessel, 5 g of2,2′-azobis(2-amidinopropane)hydrochloride initiator (i.e., V-50initiator available from Wako Pure Chemical Industries, Inc., Osaka,Japan) was dissolved in 70 g of deionized water, then the resultinginitiator solution was sparged with argon for one-half hour. A 7 gportion of the initiator solution was added to the monomer solution, andthe remainder of the initiator solution was added to the monomersolution over an hour period while heating the resulting reactionmixture to about 45° C. The reaction temperature was maintained at about45° C. for about 4 hours. The reaction mixture then was heated to 55° C.and held for two hours. Finally, 20 g of a 15% by weight aqueous V-50solution was added to the reaction mixture, and the polymerizationreaction was held at 65° C. for 12 hours to providepoly(N-vinylformamide).

The aqueous poly(N-vinylformamide) solution then was heated to about 70°C., while 20 g of a 12% by weight sodium borohydride solution (in 41%aqueous sodium hydroxide) was added to the polymer solution. After thescavenger solution was added, 480 g of 50% aqueous sodium hydroxide wasadded to the polymer solution, and the resulting solution was stirredfor about 8 hours at about 70° C. to hydrolyze thepoly(N-vinylformamide).

If desired, the resulting poly(vinylamine) solution then can be purifiedby ultrafiltration. In this optional purification, the poly(vinylamine)solution was diluted with 3 liters of deionized water. The dilutedsolution then was ultrafiltered with a 100,000 molecular weight cut-offtangential flow ultrafiltration module. The diluted polymer solution waswashed with 25 liters of deionized water, and then concentrated to 2,500ml to give a 4 wt% solution of sodium formate-free poly(vinylamine).

Example 1 was repeated, but the scavenger step using sodium borohydridewas omitted. During hydrolysis, the aqueous solution ofpoly(N-vinylformamide) gelled. Gelling was attributed to impuritiespresent in the N-vinylformamide monomer that were not removed in ascavenging step.

The following example illustrates the ability of a scavenger, likesodium borohydride, to reduce the reaction time in the synthesis of apoly(vinylamine).

EXAMPLE 2

A five liter flask was charged with 400 g of N-vinylformamide monomerand 2,970 g of deionized water, and the resulting monomer solution wassparged with argon for one hour. Separately, an initiator solution wasprepared by dissolving 5 g of V-50 in 67 g of deionized water, andsparging with argon for 0.5 hours. A portion of the initiator solution(7 g) was added to the monomer solution. The remainder of the initiatorsolution was added to the monomer solution over a one-hour time period,while the resulting reaction mixture was heated to 45° C. The reactionmixture was held at 45° C. for 2.5 hours, then heated to 55° C. and heldfor an additional 2.5 hours, and finally heated to 65° C. and held foran additional one hour. Next, 20 g of 12% sodium borohydride in a 41%aqueous sodium hydroxide solution was added to the reaction mixture,followed immediately by 480 g of a 50% aqueous sodium hydroxidesolution. The reaction mixture quickly turned pink in color but thenreturned to colorless. The hydrolysis step was continued at 70° C. foran additional 8 hours. The resulting poly(vinylamine) solution can thenbe purified, if desired, by ultra-filtration as set forth in Example 1.In the absence of a sodium borohydride scavenger, the reaction requiresan additional several hours to react all the N-vinylformamide monomersand other impurities, as set forth in Example 1.

EXAMPLE 3

Freshly distilled N-vinylformamide (250 g, 3.5 mole) and 2.8 g of 15%V-50 initiator were dissolved in 400 g of deionized water, then theresulting reaction solution was sparged with argon for 15 minutes. Next,the reaction solution was poured into a glass pan and cured at 15 mW/cm²of UV light for 25 minutes. The polymerization was exothermic,eventually reaching about 100° C. The resulting concentratedpoly(N-vinylformamide) solution was very viscous.

The concentrated poly(N-vinylformamide) solution (312 g) then wasdiluted with 2 liters of deionized water, and the diluted polymersolution was heated to 70° C. Six (6) g of a sodium borohydride solution(15% by weight of 41% aqueous sodium hydroxide) was added dropwise tothe heated polymer solution over a five-minute time period, followed bythe addition of 143 g of 50% aqueous sodium hydroxide. The resultingsolution was maintained at 70° C. for 8 hours to hydrolyze thepoly(N-vinylformamide), then cooled and purified as in Example 1.

The present improved method of manufacturing poly(vinylamine) also canbe used in processes wherein poly(vinylamine) is derived from, forexample, poly(N-vinylacetamide), poly(N-vinylpropionamide),poly(N-vinylsuccinamide), and similar N-vinylcarboxamides.

The present improved method of manufacturing a poly(vinylamine) can alsobe used in the manufacture of a crosslinked poly(vinylamine). Asdescribed above, SAPs are crosslinked to a sufficient extent such thatthe polymer is water insoluble. Crosslinking serves to render thepoly(vinylamine) polymers substantially water insoluble, and, in part,serves to determine the absorptive capacity of the polymers. For use inabsorption applications, the poly(vinylamine) is lightly crosslinked,i.e., has a crosslinking density of less than about 20%, and preferablyless than about 10%, and most preferably about 0.01% to about 7%.

When used, a crosslinking agent most preferably is included in an amountof less than about 7 wt %, and typically about 0.1 wt % to about 5 wt %,based on the total weight of monomers. A poly(vinylamine) can becrosslinked by two different pathways. One pathway utilizes olefinicallyunsaturated crosslinking monomers that copolymerize with theN-vinylamide, and, therefore, form a part of the polymeric backbone. Thecrosslinked poly(N-vinylamide) then is hydrolyzed to provide crosslinkedpolyvinylamine.

Examples of crosslinking polyvinyl monomers include, but are not limitedto, polyacrylic (or polymethacrylic) acid esters represented by thefollowing formula (I); and bisacrylamides, represented by the followingformula (II).

wherein x is ethylene, propylene, trimethylene, hexa-methylene,2-hydroxypropylene, —(CH₂CH₂O)_(n)—CH₂CH₂—, or

n and m are each an integer 5 to 40, and k is 1 or

wherein 1 is 2 or 3.

The compounds of formula (I) are prepared by reacting polyols, such asethylene glycol, propylene glycol, trimethylolpropane, 1,6-hexanediol,glycerin, pentaerythritol, polyethylene glycol, or polypropylene glycol,with acrylic acid or methacrylic acid. The compounds of formula (II) areobtained by reacting polyalkylene polyamines, such as diethylenetriamineand triethylenetetramine, with acrylic acid.

Specific crosslinking monomers include, but are not limited to,1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,3-butyleneglycol diacrylate, 1,3-butylene glycol dimethacrylate, diethylene glycoldiacrylate, diethylene glycol dimethacrylate, ethoxylated bisphenol Adiacrylate, ethoxylated bisphenol A dimelhacrylate, ethylene glycoldimethacrylate, 1,6-hexanediol diacrylate, 1,6-hexanedioldimethacrylate, neopentyl glycol dimethacrylate, polyethylene glycoldiacrylate, polyethylene glycol dimethacrylate, triethylene glycoldiacrylate, triethylene glycol dimethacrylate, tripropylene glycoldiacrylate, tetraethylene glycol diacrylate, tetraethylene glycoldimethacrylate, dipentaerythritol pentaacrylate, pentaerythritoltetraacrylate, pentaerythritol triacylate, trimethylolpropanetriacrylate, trimethylolpropane trimethacrylate,tris(2-hydroxyethyl)isocyanurate triacrylate,tris(2-hydroxyethyl)isocyanurate trimethacrylate, divinyl esters of apolycarboxylic acid, diallyl esters of a polycarboxylic acid, triallylterephthalate, diallyl maleate, diallyl fumarate,hexamethylenebismaleimide, trivinyl trimellitate, divinyl adipate,diallyl succinate, a divinyl ether of ethylene glycol, cyclopentadienediacrylate, tetraallyl ammonium halides or mixtures thereof. Compoundslike divinylbenzene and divinyl ether also can be used to crosslink thepoly(N-vinylamide). Especially preferred crosslinking agents areN,N′-methylenebisacrylamide, N,N′-methylenebismethacrylamide, ethyleneglycol dimethacrylate, and trimethylolpropane triacrylate.

The following example illustrates a crosslinked poly(vinylamine)prepared in accordance with the present invention.

EXAMPLE 4

A monomer mixture containing N-vinylformamide (250 grams), deionizedwater (250 grams), methylenebisacrylamide (1.09 grams), and V-50initiator (0.42 grams) was placed in a shallow dish, then polymerizedunder an ultraviolet lamp as set forth in Example 3 until the mixturepolymerized into a rubbery gel. The concentrated poly(N-vinylformamide)then was treated with a sodium borohydride/sodium hydroxide solution, asset forth in Example 1, to yield a lightly crosslinked poly(vinylamine).Sodium formate present in the crosslinked poly(vinylamine) can beremoved by washing the resin with acetone/water mixtures, or by othersuitable methods known to persons skilled in the art.

Poly(vinylamine) also can be crosslinked in solution by suspending ordissolving uncrosslinked poly(vinylamine) in an aqueous medium, thenadding a di- or poly-functional compound capable of crosslinking thepoly(vinylamine) by reaction with the amino groups of the polymer. Suchcrosslinking agents include, for example, multifunctional aldehydes(e.g., glutaraldehyde), multifunctional acrylates (e.g., butanedioldiacrylate, TMPTA), halohydrins (e.g., epichlorohydrin), dihalides(e.g., dibromopropane), disulfonate esters (e.g.,WS(O₂)O—(CH₂)_(n)—OF(O)₂W, wherein n is one to 10, and W is methyl ortosyl), multifunctional epoxies (e.g., ethylene glycol diglycidylether), multifunctional esters (e.g., dimethyl adipate), multifunctionalacid halides (e.g., oxalyl chloride), multifunctional carboxylic acids(e.g., succinic acid), carboxylic acid anhydrides (e.g., succinicanhydride), organic titanates (e.g., TYZOR AA from DuPont), melamineresins (e.g., CYMEL 301, CYMEL 303, CYMEL 370, and CYMEL 373 from CytecIndustries, Wayne, N.J.), hydroxymethyl ureas (e.g.,N,N′-dihydroxymethyl-4,5-dihydroxyethyleneurea), and multifunctionalisocyanates (e.g., toluene diisocyanate). Crosslinking agents also aredisclosed in Pinschmidt, Jr. et al. U.S. Pat. No. 5,085,787,incorporated herein by reference, and in EP 450 923.

In general, the crosslinking agent should be water soluble and possesssufficient reactivity with poly(vinylamine) such that crosslinkingoccurs in a controlled fashion, preferably at a temperature of about 25°C. to about 150° C. A preferred crosslinking agent is ethylene glycoldiglycidyl ether (EGDGE), a water-soluble diglycidyl ether.

The following example illustrates light crosslinking of a sodiumformate-free poly(vinylamine) of the present invention using apolyfunctional crosslinking agent that reacts with the amino groups ofthe polymer.

EXAMPLE 5

To 2 liters of a 3% by weight aqueous poly(vinylamine) solution wasadded 0.18 g of ethyleneglycol diglycidyl ether (EGDGE). The resultingmixture was stirred to dissolve the EGDGE, then the mixture was heatedto about 60° C. and held for one hour to gel. The gel was heated toabout 80° C. and held until about 90% of the water was removed. Theresulting gel then was extruded and dried to a constant weight at 80° C.The dried, lightly crosslinked poly(vinylamine) then was cryogenicallymilled to form a granular material capable of absorbing water or acidsolutions. The gel exhibited the following absorption characteristics in0.1 M hydrochloric acid (HCl):

AUNL¹⁾=59.3 g/g

AUL²⁾ (0.28 psi)=37.8 g/g

AUL²⁾ (0.7 psi)=26.4 g/g

¹Absorption under no load; and

²Absorption under load.

Absorption under load (AUL) is a measure of the ability of an SAP toabsorb fluid under an applied pressure. The AUL was determined by thefollowing method, as disclosed in U.S. Pat. No. 5,149,335, incorporatedherein by reference.

An SAP (0.160 g +/−0.001 g) is carefully scattered onto a 140-micron,water-permeable mesh attached to the base of a hollow plexiglasscylinder with an internal diameter of 25 mm. The sample is covered witha 100 g cover plate and the cylinder assembly weighed. This gives anapplied pressure of 20 g/cm² (0.28 psi). Alternatively, the sample canbe covered with a 250 g cover plate to give an applied pressure of 51g/cm² (0.7 psi). The screened base of the cylinder is placed in a 100 mmpetri dish containing 25 milliliters of a test solution (usually 0.9%saline), and the polymer is allowed to absorb for 1 hour (or 3 hours).By reweighing the cylinder assembly, the AUL (at a given pressure) iscalculated by dividing the weight of liquid absorbed by the dry weightof polymer before liquid contact. As discussed hereafter, thepoly(vinylamine) particles also can be surface treated with acrosslinking agent, like ethyleneglycol diglycidyl ether, to give anabsorbent having improved performance under external pressure.

In a preferred embodiment, a lightly crosslinked poly(vinylamine) issubjected to a process step wherein the surface of thepoly(N-vinylamine) is further crosslinked. It has been found thatsurface crosslinking of a poly(vinylamine) enhances the ability of thepolymer to absorb and retain aqueous media under load.

Surface crosslinking is achieved by spraying poly(vinylamine) particleswith an isopropyl alcohol solution of a surface crosslinking agent towet predominantly only the outer surfaces of the poly(vinylamine)particles. Surface crosslinking and drying of the polymer then isperformed, preferably by heating at least the wetted surfaces of thepoly(vinylamine) particles.

Typically, the poly(vinylamine) particles are surface treated with analcoholic solution of a surface crosslinking agent. The particles can bein the form of granules, a foam, beads, flakes, fibers, or powders, forexample. The solution contains about 0.01% to about 4%, by weight,surface crosslinking agent, and preferably about 0.4% to about 2%, byweight, surface crosslinking agent in a suitable solvent. The solutioncan be applied as a fine spray onto the surface of freely tumblingpoly(vinylamine) particles at a ratio of about 1:0.01 to about 1:0.5parts by weight poly(vinylamine) to solution of surface crosslinkingagent. The surface crosslinker is present in an amount of 0% to about1%, by weight of the poly(vinylamine), and preferably 0% to about 0.5%by weight. To achieve the full advantage of the present invention, thesurface crosslinker is present in an amount of about 0.001% to about0.1% by weight.

The crosslinking reaction and drying of the surface-treatedpoly(vinylamine) particles are achieved by heating the surface-treatedpolymer at a suitable temperature, e.g., about 25° C. to about 150° C.,and preferably about 105° C. to about 120° C. However, any other methodof reacting the crosslinking agent to achieve surface crosslinking ofthe poly(vinylamine) particle, and any other method of drying thepoly(vinylamine) particles, such as microwave energy, or the like, canbe used.

Suitable surface crosslinking agents include the di- or poly-functionalmolecules capable of reacting with amino groups and crosslinkingpoly(vinylamine). Preferably, the surface crosslinking agent is alcoholor water soluble and possesses sufficient reactivity with apoly(vinylamine) such that crosslinking occurs in a controlled fashionat a temperature of about 25° C. to about 150° C.

Nonlimiting examples of suitable surface crosslinking agents include:

(a) dihalides and disulfonate esters, for example, compounds of theformula

Z—(CH₂)_(p)—Z

wherein p is a number from 2 to 12, and Z, independently, is halo(preferably bromo), tosylate, mesylate, or other alkyl or aryl sulfonateesters;

(b) multifunctional aziridines;

(c) multifunctional aldehydes, for example, glutaraldehyde, trioxane,paraformaldehyde, terephthaldehyde, malonaldehyde, and glyoxal, andacetals and bisulfites thereof;

(d) halohydrins, like epichlorohydrin;;

(e) multifunctional epoxy compounds, for example, ethylene glycoldiglycidyl ether, bisphenol A diglycidyl ether, and bisphenol Fdiglycidyl ether,

(f) multifunctional carboxylic acids and esters, acid chlorides, andanhydrides derived therefrom, for example, di- and poly-carboxylic acidscontaining two to twelve carbon atoms, and the methyl and ethyl esters,acid chlorides, and anhydrides derived therefrom, like oxalic acid,adipic acid, succinic acid, dodecanoic acid, malonic acid, and glutaricacid, and esters, anhydrides, and acid chlorides derived therefrom;

(g) organic titanates, like TYZOR AA, available from E.I. DuPont deNemours, Wilmington, Del.;

(h) melamine resins, like the CYMEL resins available from CytecIndustries, Wayne, N.J.;

(i) hydroxymethyl ureas, like N,N′-dihydroxymethyl-4,5-dihydroxyethyleneurea; and

(j) multifunctional isocyanates, like toluene diisocyanate, isophoronediisocyanate, xylene diisocyanate, and hexamethylene diisocyanate.

A preferred surface crosslinking agent is ethylene glycol diglycidylether (EGDGE), which is a water-soluble diglycidyl ether whichcrosslinks poly(vinylamine) at a temperature of about 25° C. to about150° C.

The following Example 6 illustrates surface treatment and crosslinkingof a lightly crosslinked poly(vinylamine).

EXAMPLE 6

Divinylbenzene crosslinker (1.085 g, 55% active, by weight, instyrene/ethylstyrene), aqueous V-50 initiator (2.541 g, 15%), andN-vinylformamide (245 g, 3.45 moles) were mixed in 350 g of deionizedwater. The resulting solution was sparged with argon for 15 minutes, andthen polymerized under UV light (15 mW/cm²) for one hour. The resultinggel was extruded, dried at 100° C., and milled to produce particles oflightly crosslinked poly(vinylamine).

A portion of the poly(N-vinylformamide) particles (82 g) was hydrolyzedby dispersing the particles in a solution containing 168 g cyclohexane,112 g 1-butanol, and 46 g of powdered sodium hydroxide. The resultingsuspension then was heated at about 70° C. for about 6 hours. Next, 150g of deionized water was added to the suspension, and the organicsolvents were decanted. Acetone (230 g) then was added to collapse thegel and remove the sodium formate by-product. The water/acetone wash wasrepeated three more times, and the gel was dried then remilled. Theresulting poly(vinylamine) gel then was surface treated with ethyleneglycol diglycidyl ether at various levels, and dried at 145° C. toprovide a surface crosslink.

The poly(vinylamine) then was tested for an ability to absorb and retain0.1 M hydrochloric acid.

TABLE 1 Surface crosslink AUNL¹ and AUL² (0.1 M HCl) Level (ppm)³ NoLoad 0.28 psi 0.7 psi  0 51 23 9.9 100 47 27 19 500 47 27 19 1000  46 2820 2000  41 26 20 ³ppm--parts per million of surface crosslinker.

The absorption data shows that surface crosslinking substantiallyimproves the absorption under load of a poly(vinylamine), especially ata load of 0.7 psi.

(b) Poly(vinylamine)-Based SAPs

Poly(vinylamine) typically does not function as an SAP in its neutralform because there is no ionic charge on the polymer. The driving forcefor water absorption and retention therefore is lacking. However, whenconverted to a salt, or used in conjunction with an acidicwater-absorbing resin, like a polyacrylic acid, a poly(vinylamine) thenbehaves likes an SAP. It should be understood that a poly(vinylamine)produced either by the above-described improved method, or by a prior,conventional method, can be used in a poly(vinylamine)-based SAP.

(i) Salts of Poly(vinylamine)

As previously discussed, sodium poly(acrylate) is considered the bestSAP, and, therefore, is the most widely used SAP in commercialapplications. Sodium poly(acrylate) has polyelectrolytic properties thatare responsible for its superior performance in absorbent applications.These properties include a high charge density, and charge relativelyclose to the polymer backbone.

Poly(vinylamine) is a neutral polymer, and, accordingly, does notpossess the polyelectrolytic properties necessary to provide an SAP.However, poly(vinylamine) salts have polyelectrolytic propertiessufficient to provide an SAP. The poly(vinylamine) used to provide anSAP is a lightly crosslinked poly(vinylamine), and preferably is surfacecrosslinked, as set forth above.

Such lightly crosslinked, and optionally surface crosslinked,poly(vinylamine) polymers are converted into salts by methods known inthe art. For example, the preparation of poly(vinylamine HCl) by theaddition of hydrochloric acid to a poly(vinylamine) is set forth inPinschmidt, Jr. et al. U.S. Pat. No. 5,085,787, and in Gless, Jr. et al.U.S. Pat. No. 4,018,826, or by hydrolysis of a poly(N-vinylamide) withhydrochloric acid.

A poly(vinylamine) salt useful as an SAP, however, is not limited to thehydrochloride salt. Poly(vinylamine) can be reacted with a variety ofacids to provide a poly(vinylamine) salt useful as an SAP, but thepreferred acids are mineral acids. To achieve the full advantage of thepresent invention, the poly(vinylamine) salt is a hydrochloride salt.

To demonstrate the ability of a poly(vinylamine) salt to act as an SAP,the lightly crosslinked poly(vinylamine) of Example 5 was converted tothe hydrochloride salt by methods well known in the art. Thepoly(vinylamine) salt was tested for its ability to absorb and retaindeionized water and electrolyte-containing aqueous media (i.e., 0.9% byweight aqueous sodium chloride).

In particular, poly(vinylamine) samples, as prepared in Example 5, wereconverted to the hydrochloride salt using different amounts of 1Nhydrochloric acid (HCl). The resulting gels of poly(vinylamine) saltthen were dried and evaluated for an ability to absorb a 0.9% by weightaqueous NaCl solution. The results are summarized in Table 2.

TABLE 2 AUL² AUL² Mole % HCL⁴ AUNL¹ (0.28 psi) (0.7 psi)  0 18.7 13.712.6 30 31.6 21.5 15.9 50 39.8 25.6 20.1 70 43.0 23.4 13.5 100  28.5 9.1  9.5 ⁴mole % HCl added to the poly(vinylamine) based on the molesof N-vinylformamide monomer used to prepare the poly(vinylamine).

The absorbency results summarized in Table 2 show that absorbencyincreases dramatically, both under load and under no load, when thepoly(vinylamine) is converted to a hydrochloride salt, especially in therange of about 30 to about 70 mole % conversion to the salt. Inaccordance with an important feature of the present invention, apoly(vinylamine) exhibits the properties of an SAP when converted to asalt in an amount of about 10 to about 100, and preferably about 20 toabout 90, mole percent. To achieve the full advantage of the presentinvention, the poly(vinylamine) is converted to a salt in an amount ofabout 25 to about 75 mole %, based on the weight of N-vinylamide monomerused to prepare the poly(vinylamine).

In another test, a lightly crosslinked poly(vinylamine), as prepared inExample 6, was surface treated with various levels of ethylene glycoldiglycidyl ether (EGDGE) in isopropyl alcohol, followed by drying andcuring at 80° C. The surface crosslinked granules of lightly crosslinkedpolyvinylamine then were partially neutralized (i.e., 50 mole %) with 1NHCl. The surface crosslinked polyvinylamine salt, then was tested for anability to absorb and retain a 0.9% aqueous NaCl solution. The resultsare summarized in Table 3, and show that a neutralized and surfacecrosslinked poly(vinylamine) shows an improvement in AUL.

TABLE 3 Surface crosslink Level of EGDGE AUL² AUL² (ppm)⁴ AUNL¹ (0.28psi) (0.7 psi)  0 35.8 16.6 9.3 100 35.3 18.9 11.3 500 31.5 16.3 11.21000  31.3 17.8 11.5 2000  28.8 18.0 11.9

(ii) Poly(vinylamine) in SAPs

As illustrated above, poly(vinylamine), in its free base form, does notfunction as an SAP for neutral-to-basic aqueous media. Similarly,polyacrylic acid, in its free acid form, does not function as an SAP forneutral-to-acidic aqueous media. In each case, the polymer has a lowcharge density, and, accordingly, a major driving force for absorptionand retention, i.e., electrostatic repulsion, is missing. In contrast,partially neutralized polyacrylic acid has a sufficient charge density,and is currently used as an SAP by itself. Similarly, as disclosedabove, poly(vinylamine) salts have a high charge density and areexcellent SAPs.

However, a poly(vinylamine), in its free base form, can act as anabsorbent for acidic aqueous media, i.e., media having a pH less than 7,as illustrated in Examples 5 and 6, wherein one gram of poly(vinylamine)absorbed 59.3 g and 51 g of 0.1 M hydrochloric acid under no load,respectively. The acidic media protonates the amino groups of thepoly(vinylamine), thereby providing sufficient charge density for theprotonated poly(vinylamine) to perform as an SAP. Accordingly,poly(vinylamine), by itself, can be used to absorb acidic aqueous media,for example, to absorb an acid spill.

It also has been found that poly(vinylamine) polymers, in their freebase form, are useful components in superabsorbent materials furthercontaining an acidic water-absorbing resin. For example, asuperabsorbent material of the present invention is an admixture of apoly(vinylamine) and an acidic water-absorbing resin, like polyacrylicacid. The present superabsorbent materials are particularly useful withrespect to absorbing and retaining aqueous media containingelectrolytes.

Currently, superabsorbent materials containing two absorbing components,i.e., bi-component SAP materials, are being investigated as an improvedclass of SAPs. Typically, one component is a water-absorbing resin, andthe second component acts in an ion exchange capacity to removeelectrolytes from an aqueous media.

In contrast, the present invention is directed to a bi-component SAPmaterial comprising two uncharged, slightly crosslinked polymers, eachof which is capable of swelling and absorbing aqueous media. Whencontacted with water, the two uncharged polymers neutralize each otherto form a superabsorbent material. Neither polymer in its uncharged formbehaves as an SAP by itself when contacted with water. The presentbi-component superabsorbent material, therefore, contains two resins,one acidic and one basic, which are capable of acting as an absorbentmaterial in their polyelectrolyte form. While polyacrylic acid is anexcellent choice for the acidic resin, until the present invention,there has not been an adequate basic resin.

Therefore, in accordance with an important feature of the presentinvention, poly(vinylamine) is used as the basic resin for abi-component SAP material. The poly(vinylamine) is lightly and thepoly(vinylamine) particles preferably are surface crosslinked to improveabsorbency characteristics. The poly(vinylamine) and acid resincombination behaves like an SAP in the presence of water, and especiallybrackish water. The poly(vinylamine) can be prepared by the improvedmethod disclosed herein, or by prior methods known in the art.Crosslinking and surface crosslinking can be performed as set forthabove.

The poly(vinylamine) is a basic resin that is admixed with an acidicresin. The acidic resin can be any resin that acts as an SAP in itsneutralized form. The acidic resin typically contains a plurality ofcarboxylic acid, sulfonic acid, phosphonic acid, phosphoric acid, orsulfuric acid moieties, or a mixture thereof.

Examples of acidic resins include, but are not limited to, polyacrylicacid, hydrolyzed starch-acrylonitrile graft copolymers, starch-acrylicacid graft copolymers, saponified vinyl acetate-acrylic estercopolymers, hydrolyzed acrylonitrile copolymers, hydrolyzed acrylamidecopolymers, ethylenemaleic anhydride copolymers, isobutylene-maleicanhydride copolymers, poly(vinylsulfonic acid), poly(vinyl-sulfuricacid), poly(vinylphosphoric acid, sulfonated polystyrene,poly(vinylphosphonic) acid, and mixtures thereof. The preferred acidicresins are the polyacrylic acids.

The poly(vinylamine) is present in its uncharged, i.e., free base, form,and the acidic resin is present in its free acid form. It is envisionedthat a low percentage, i.e., 25% or less, of the amine and acidfunctionalities can be in their charged form, due to processing, forexample. The low percentage of charged functionalities does notadversely affect performance of the superabsorbent material, but theamount of charged functionalities should be minimized.

The poly(vinylamine) and acidic resin are admixed in a weight ratio ofabout 5:95 to about 95:5, and preferably about 10:90 to about 90:10. Toachieve the full advantage of the present invention, the resins areadmixed in a weight ratio of about 30:70 to about 70:30. A presentbi-component SAP material is prepared by simply admixing particles ofthe poly(vinylamine) and acidic resin to provide a uniform particulatematerial

To illustrate a present bi-component SAP material, the followingexamples were prepared and tests performed:

EXAMPLE 7

Powdered poly(vinylamine), as prepared in Example 5 (particle size210-710 μm) was admixed with lightly crosslinked polyacrylic acid(particle size 210-710 μm, 0% neutralized) in a weight ratio of 37%poly(vinylamine) to 63% polyacrylic acid. The absorbency characteristicsof the resulting bi-component SAP were tested and compared to theabsorbency characteristics with respect to a 0.9% by weight aqueous NaClsolution. The results are set forth in Table 4.

TABLE 4 AUL AUL AUL AUL (0.28 (0.7 (0.28 (0.7 psi, psi, AUNL psi, psi,AUNL 1 hr.) 1 hr.) (1 hr.) 3 hr.) 3 hr.) (3 hr.) Poly(vinyl- 21.2 18.628.3 23.8 20.5 36.3 amine)/ Polyacrylic Acid Blend Poly(vinyl- 14.2 14.421.4 15 14.3 23.4 amine)

Table 4 shows that the poly(vinylamine)/polyacrylic acid blend hassubstantially improved absorption properties compared topoly(vinylamine) alone.

The bi-component SAP materials are especially useful in articlesdesigned to absorb and retain liquids, especially electrolyte-containingliquids. Such articles include, for example, diapers and catamenialdevices.

To illustrate the improved absorption properties of a bi-component SAPmaterial of the present invention, the blends described in the followingTable 5 were prepared and tested for an ability to absorb syntheticurine under a 0.7 psi load. As used here and throughout thespecification, poly(AA) (DN=70) refers to a standard, commercialpoly(AA) neutralized about 70% to about 80%, and poly(AA) (DN=0) refersto unneutralized poly(AA).

TABLE 5 Blend AUL 0.7 AUL 0.7 Example Ratio⁵ psi (1 hr) psi (3 hr)  1¹75/25 27.1 28.9 1 50/50 30.9 33 1 25/75 35.9 40.2  2² 75/25 26.6 27.3 250/50 28.7 30.3 2 25/75 26.3 27.3  3³ 75/25 25.3 26 3 50/50 21.3 22.8 325/75 15.7 16.4  4⁴ 55/45 37 45.2 ¹Blend of (a) partially neutralizedpoly(AA)-(DN = 70) and (b) a mixture containing 55% by weightpoly(vinylamine) and 45% by weight poly(AA) (DN = 0); ²Blend of (a)partially neutralized poly(AA)-(DN = 70) and (b) poly(vinylamine);³Blend of (a) partially neutralized poly(AA)-(DN = 70) and (b) poly(AA)(DN = 0); ⁴Blend of (a) poly(vinylamine) and (b) poly(AA)-(DN = 0); and⁵Weight ratio of (a) to (b) in each blend.

The data presented in Table 5 shows a substantial improvement inabsorption properties achieved by a bi-component SAP material of thepresent invention, either when used alone (i.e., Example 4) or incombination with a standard SAP material, like poly(AA) (DN=70) (i.e.,Example 1).

To further illustrate that the present bi-component SAP materials havean improved ability to absorb and retain liquids compared to apresent-day SAP, laboratory diaper cores containing a presentbi-component SAP material were prepared and compared to laboratorydiaper cores containing a conventional SAP. In particular, the followingthe diaper cores were prepared:

Core A1—50% poly(AA) (DN=70) and 30% fluff

Core A2—70% poly(AA) (DN=70) and 50% fluff

Core B—27.5% poly(vinylamine), 22.5% poly(AA) (DN=0), and 50% fluffpulp, by weight,

Core C—identical to Core B except the poly(vinylamine) was surfacecrosslinked with 500 ppm EGDGE,

Core D—38.5% poly(vinylamine), 31.5% poly(AA) (DN=0), and 30% fluffpulp, by weight,

Core E—identical to Core D except the poly(vinylamine) was surfacecrosslinked with 500 ppm EGDGE.

Typically, commercial diapers contain 45% to 60% by weight of a pulp toachieve rapid absorption of a liquid. Diaper Cores A through E werecompared to one another to illustrate the improved permeability andabsorption rate, and improved liquid absorption and retention, providedby a diaper having a core that contains a bi-component SAP material ofthe present invention.

Present day diapers generally consist of a topsheet made from a nonwovenmaterial that is in contact with the skin of the wearer, an acquisitionlayer below (i.e., opposite the skin of wearer) the topsheet, a corethat is below the acquisition layer, and a backsheet below the core.This construction is well known in the industry.

Cores A through E were prepared using a conventional laboratoryprocedure as follows:

A laboratory core-forming unit comprising a two-chamber vacuum systemforms an airlaid fluff pulp-absorbent composite matrix to produce a 12cm×21 cm diaper core. The core-forming unit comprises a roller brush ona variable-speed laboratory motor, a fiber distribution screen in closeproximity to the brush, a forming screen on an adjustable damper, and avacuum system capable of supplying a consistent and continuous negativepressure between 8 and 15 inches of water.

The core-forming unit is contained such that the vacuum pulls the fibersand granular material from an adjustable introduction slide, through therotating brush and distribution screen, directly onto the formingscreen. The vacuum exhaust is recirculated through the inlet of theformation slide, thereby controlling the temperature and humidity of theoperation.

When forming a core, the desired amount of defiberized fluff pulp isevenly disbursed in small pieces onto the brush roller in the upperchamber. In the lower chamber, a rectangular tissue, or topsheet (21cm×12 cm), is placed onto the forming screen. For most cores, thesliding upper chamber lid is partially closed to leave about a one-halfinch gap. In the case of a homogeneous pulp/SAP core, the SAP issprinkled through the gap into the upper chamber immediately after thebrush begins rotating. In order to achieve a homogeneous distribution, asmall amount of SAP is added to the fluff prior to beginning the motor.The amount of time used to introduce the remainder of the SAP varieswith the amount of fluff pulp utilized. After the fiber and absorbentpolymer material are deposited, the motor is turned off, and the damperunit containing the laboratory core is removed from the lower chamber.The uncompressed core then is placed on a backsheet made from apolymeric film, and put into a compression unit. At this time, anotherrectangular tissue and a nonwoven coverstock is placed on top of thecore. Absorbent cores are compressed for a given amount of time,typically 5 minutes, with a hydraulic press at pressures of betweenabout 5,000 psi and about 10,000 psi, and typically about 7,000 psi, toachieve the desired density. After the 5 minutes, thelaboratory-prepared absorbent cores are removed from the press, weighed,and measured for thickness.

Cores A through H2, and other laboratory cores and commercial diapers,were tested for rewet under a 0.7 psi load, liquid acquisition time, andliquid acquisition rate. The following describes the procedure todetermine the acquisition and rewet under load of a hygienic article,like a diaper. These tests exhibit the rate of absorption and fluidretention of a 0.9%, by weight, saline solution, by a hygienic articleover 3 to 5 separate fluid insults while under a load of 0.7 psi.

Apparatus

100 ml separatory funnel, configured to deliver a flow rate of 7ml/sec., or equivalent;

3.642 kg circular weight (0.7 psi) 10 cm diameter, with 2.38 cm IDperspex dose tube through the center of weight;

VWR Scientific, 9 cm filter paper or equivalent;

2.5 kg circular weight (0.7 psi)-8 cm diameter;

Digital timer;

Electronic balance (accuracy of a 0.01 gram);

Stopwatch.

Procedure

1. Preparation

(a) Record the weight (g) of the hygienic article, e.g., diaper, to betested;

(b) Place hygienic article flat on the bench top, for example, byremoving any elastics and/or taping the ends of the article to the benchtop;

(c) Place the 3.64 kg circular weight onto the hygienic article with theopening of the perspex dose tube positioned at the insult point (i.e., 5cm toward the front from the center).

2. Primary Insult and Rewet Test

(a) Measure 100 ml of 0.9% NaCl solution (i.e., 0.9% by weight sodiumchloride in deionized or distilled water) into separatory funnel.Dispense the NaCl solution into the perspex tube of the weight at a flowrate of 7 ml/sec and start the timer immediately. Stop the timer whenall of the NaCl solution has completely disappeared from the surface ofthe hygienic article at the base of the perspex tube. Record this timeas the primary acquisition time (sec).

(b) After 10 minutes have elapsed, remove the weight and conduct therewet test procedure:

(i) Weigh a stack of 10 filter papers, record this value (dry weight).

(ii) Place the filter papers over the insult point on the hygienicarticle. Set the timer for 2 minutes. Place the 2.5 kg weight onto thefilter papers and start timer immediately.

(iii) After 2 minutes have elapsed, remove the weight and reweigh thefilter papers (wet weight). Subtract the dry weight of the filter papersfrom the wet weight, this is the rewet value. Record this value as theprimary rewet value (g).

3. Secondary Insult and Rewet Test

(a) Place the 3.64 kg weight back onto the hygienic article in the sameposition as before. Repeat step 2a using 50 ml NaCl solution (recodingthe absorption time as the secondary acquisition time) and steps 2b(i)-(iii) using 20 filter paper (recording the rewet values as thesecondary rewet).

4. Tertiary, and additional, Insult and Rewet Tests

(a) Place the load back onto the diaper in the same position as before.Repeat step 2a using 50 ml of NaCl solution (recording the absorptiontime as the tertiary acquisition time) and steps 2b (i)-(iii) using 30filter paper (recording the rewet value as the tertiary or subsequentrewet).

FIGS. 1-4 illustrate the improved properties of diapers that contain abi-component SAP material of the present invention.

FIG. 1 contains plots of acquisition time vs. number of insults with0.9% aqueous saline, under a 0.7 psi load, for diapers containing CoresA1, B, and C. In FIG. 1, Cores B and C are cores of the presentinvention. Core A1 is a comparative core.

Cores B and C exhibited an excellent ability to acquire 0.9% salineunder a 0.7 psi load. Core A1 acquired the saline relatively slowly,especially during rewetting. Core A1 represents a standard corecontaining 50% SAP, and has a higher acquisition time than Cores B or C.The acquisition time for Core A1 could not be measured beyond the fourthinsult because acquisition was very slow.

FIG. 2 illustrates the acquisition time for Cores A2, D, and E. FIG. 2shows that Cores D and E, i.e., cores of the present invention,significantly outperform a comparative laboratory core (Core A2) withrespect to acquisition time of 0.9% saline under a 0.7 psi load.

FIGS. 3 and 4 illustrate that a core of the present invention, i.e.,Cores B-E, have a greater acquisition rate than comparative laboratorycores containing poly(AA) (DN=70).

Overall, the data presented in FIGS. 1-4 demonstrate that a diaper coreof the present invention maintains a flat, essentially constant, orsurprisingly a decreased, acquisition time over five insults, whereasprior cores demonstrate an increased acquisition time over prior cores.The data also shows an improved acquisition rate as the number ofinsults increases. Such results are unexpected because prior coresexhibit a decrease in acquisition rate as the number of insultsincrease, especially between the first and second insult. The presentcores maintain the initial acquisition rate after a second insult withsaline (see FIG. 3). The present cores, therefore, can have a secondacquisition rate that is at most 20% slower than a first acquisitionrate, and preferably is at least equal to the first acquisition rate.The practical result of these improved properties is a core having animproved ability to prevent leakage in gush situations and in rewetsituations.

The data shows that improvements in liquid absorption, both with respectto kinetics and retention, are observed if the standard poly(AA) (DN=70)presently used in cores is completely replaced by bi-component SAPmaterial of the present invention.

The improved results demonstrated by a core of the present inventionalso permit the thickness of the core to be reduced. Typically, corescontain 50% or more fluff or pulp to achieve rapid liquid absorptionwhile avoiding problems like gel blocking. The present cores, whichcontain a bi-component SAP material, acquire liquids sufficiently fastto reduce problems, like gel blocking, and, therefore, the amount offluff or pulp in the core can be reduced. A reduction in the amount ofthe low-density fluff results in a thinner core, and, accordingly, athinner diaper.

Cores containing as little as about 25% by weight of a bi-component SAPmaterial demonstrate an excellent ability to absorb and retain liquids.Preferably, a core of the present invention contains at least 50% of abi-component SAP material, and most preferably at least 75% of abi-component SAP material. The bi-component SAP material can be usedalone in the core, or in combination with fluff and/or standard SAPparticles, like poly(AA) (DN=70). The bi-component SAP material can beadmixed with the fluff and/or standard SAP particles for introductioninto a diaper core. Alternatively, the diaper core can contain zones ofbi-component SAP particles and zones of standard SAP particles.

Many modifications and variations of the invention as hereinbefore setforth can be made without departing from the spirit and scope thereofand, therefore, only such limitations should be imposed as are indicatedby the appended claims.

What is claimed is:
 1. A diaper having a core, said core comprising atleast 25% by weight of a superabsorbent material comprising (a) alightly crosslinked poly(vinylamine), and (b) an acidic water-absorbingresin selected from the group consisting of polyacrylic acid, ahydrolyzed starch-acrylonitrile graft copolymer, a starch-acrylic acidgraft copolymer, a saponified vinyl acetate-acrylic ester copolymer, ahydrolyzed acrylonitrile copolymer, a hydrolyzed acrylamide copolymer,an ethylene-maleic anhydride copolymer, an isobutylene-maleic anhydridecopolymer, a poly(vinylsulfonic acid), a poly(vinylsulfuric acid), apoly(vinylphosphoric acid), a sulfonated polystyrene, apoly(vinylphosphonic acid), and mixtures thereof, wherein thepoly(vinylamine) and the acidic resin are present in a weight ratio ofabout 5:95 to about 95:5, and the poly(vinylamine) is neutralized 0 to25% and the acidic water-absorbing resin is neutralized 0 to 25%.
 2. Thediaper of claim 1 wherein the core comprises at least 50% by weight ofthe superabsorbent material.
 3. The diaper of claim 1 wherein the corecomprises at least 75% by weight of the superabsorbent material.
 4. Thediaper of claim 1 wherein the core further comprises an acidicwater-absorbing resin neutralized from 25 to 100%.
 5. The diaper ofclaim 4 wherein the neutralized acidic water-absorbing resin ispoly(acrylic acid).
 6. The diaper of claim 1 wherein the core has anacquisition rate for absorbing 100 milliliters of 0.9% saline under aload of 0.7 psi of greater than one milliliter/second.
 7. The diaper ofclaim 6 wherein the core has an acquisition rate for absorbing asubsequent 50 milliliters of 0.9% saline of greater than 0.8milliliter/second.
 8. The diaper of claim 1 further comprising atopsheet in contact with a first surface of the core, and a backsheet incontact with a second surface of the core, said second core surfaceopposite from said first core surface.
 9. The diaper of claim 8 furthercomprising an acquisition layer disposed between the topsheet and thecore.
 10. The diaper of claim 1 wherein the poly(vinylamine) is surfacecrosslinked.
 11. The diaper of claim 10 wherein the poly(vinylamine) issurface crosslinked with up to about 1% of a surface crosslinking agent,by weight of the poly(vinylamine).
 12. The diaper of claim 10 whereinthe surface crosslinker comprises ethylene glycol diglycidyl ether. 13.The diaper of claim 1 wherein the poly(vinylamine) and the acidic resinare present in a weight ratio of about 20:80 to about 80:20.
 14. Thediaper of claim 1 wherein the poly(vinylamine) and the acidic resin arepresent in a weight ratio of about 35:65 to about 65:35.
 15. The diaperof claim 1 wherein the poly(vinylamine) is a homopolymer.
 16. The diaperof claim 1 wherein the poly(vinylamine) comprises vinylamine and atleast one additional monoethylenically unsaturated monomer.
 17. A diaperhaving a core, said core comprising at least 25% by weight of asuperabsorbent material comprising (a) a lightly crosslinkedpoly(vinylamine), and (b) an acidic water-absorbing resin selected fromthe group consisting of polyacrylic acid, a hydrolyzedstarch-acrylonitrile graft copolymer, a starch-acrylic acid graftcopolymer, a saponified vinyl acetate-acrylic ester copolymer, ahydrolyzed acrylonitrile copolymer, a hydrolyzed acrylamide copolymer,an ethylene-maleic anhydride copolymer, an isobutylene-maleic anhydridecopolymer, a poly(vinylsulfonic acid), a poly(vinylsulfuric acid), apoly(vinylphosphoric acid), a sulfonated polystyrene, apoly(vinylphosphonic acid), and mixtures thereof, wherein the core hasan acquisition rate for absorbing 100 milliliters of 0.9% saline under aload of 0.7 psi of greater than one milliliter/second, and thepoly(vinylamine) is neutralized 0 to 25% and the acidic water-absorbingresin is neutralized 0 to 25%.
 18. The diaper of claim 17 wherein thecore has an acquisition rate for absorbing a subsequent 50 millilitersof 0.9% saline of greater than 0.8 milliliter/second.
 19. A diaperhaving a core, said core comprising at least 25% by weight of asuperabsorbent material comprising (a) a lightly crosslinkedpoly(vinylamine), and (b) an acidic water-absorbing resin selected fromthe group consisting of polyacrylic acid, a hydrolyzedstarch-acrylonitrile graft copolymer, a starch-acrylic acid graftcopolymer, a saponified vinyl acetate-acrylic ester copolymer, ahydrolyzed acrylonitrile copolymer, a hydrolyzed acrylamide copolymer,an ethylene-maleic anhydride copolymer, an isobutylene-maleic anhydridecopolymer, a poly(vinylsulfonic acid), a poly(vinylsulfuric acid), apoly(vinylphosphoric acid), a sulfonated polystyrene, apoly(vinylphosphonic acid), and mixtures thereof, wherein thepoly(vinylamine) is surface crosslinked with up to about 1% of a surfacecrosslinking agent, by weight of the poly(vinylamine), and wherein thepoly(vinylamine) is neutralized 0 to 25% and the acidic water-absorbingresin is neutralized 0 to 25%.
 20. The diaper of claim 19 wherein thesurface crosslinker comprises ethylene glycol diglycidyl ether.
 21. Adiaper having a core, said core comprising at least 25% by weight of asuperabsorbent material comprising (a) a lightly crosslinkedpoly(vinylamine), and (b) an acidic water-absorbing resin selected fromthe group consisting of polyacrylic acid, a hydrolyzedstarch-acrylonitrile graft copolymer, a starch-acrylic acid graftcopolymer, a saponified vinyl acetate-acrylic ester copolymer, ahydrolyzed acrylonitrile copolymer, a hydrolyzed acrylamide copolymer,an ethylene-maleic anhydride copolymer, an isobutylene-maleic anhydridecopolymer, a poly(vinylsulfonic acid), a poly(vinylsulfuric acid), apoly(vinylphosphoric acid), a sulfonated polystyrene, apoly(vinylphosphonic acid), and mixtures thereof, wherein thepoly(vinylamine) comprising vinylamine and at least one additionalmonoethylenically unsaturated monomer, and wherein the poly(vinylamine)is neutralized 0 to 25% and the acidic water-absorbing resin isneutralized 0 to 25%.